Estimate of the Absolute Value of Regional Stress prior to the 2000 Western Tottori Earthquake from Rotation of Focal Mechanisms

On October 6, 2000, an earthquake (Mw=6.6) occurred in western Tottori Prefecture, Japan in an area where no active fault had been previously identified. There was no displacement on the surface but seismic observations showed that the event was a left-lateral strike-slip fault on a northwest striking plane with a length of about 20 kilometers and slip of about 1.6 meters. A temporary short-period network of over 50 seismographs was installed soon after the mainshock in a cooperative effort by universities in Japan. The dense spacing of stations in the aftershock region enabled us to determine well-constrained P-wave first motion focal mechanisms. The results of determining the focal mechanisms showed a bimodal distribution of the P-axes in two azimuths. One of azimuths was in the east-west direction. This is similar to the P-axes direction for the mainshock and previous seismicity and is interpreted to represent the regional stress direction prior to the earthquake. The second azimuth has a more northwest direction. We interpret that the rotation of the P-axes for these aftershocks is caused by the change in stress field due to fault slip during the mainshock. We estimated the level of absolute of stress prior to Tottori earthquake by using a fault model for the earthquake and calculating the amount of stress change at the locations that the rotation in P-axes were observed. Comparing the calculated change in stress due to the earthquake with the rotation of the P-axis gives us an estimate of the level of absolute stress prior to the mainshock. The focal mechanisms of aftershocks showed a rotation of the P-axes of about 15 to 25 degrees. This relatively large change indicates that the level of stress prior to the earthquake was rather low, on the order of a few tens of MPa.